The important thing to know about a system call is its prototype.
You need to know how many arguments, their types, and the function
return type. There are seven macros that make the actual call into
the system easier. They have the form:
_syscallX(type,name,type1,arg1,type2,arg2,...)
where
X is 0–6, which are the number of arguments taken by the sys‐
tem call
type is the return type of the system call
name is the name of the system call
typeN is the Nth argument's type
argN is the name of the Nth argument
These macros create a function called name with the arguments you
specify. Once you include the _syscall() in your source file, you
call the system call by name.

Starting around kernel 2.6.18, the _syscall macros were removed from
header files supplied to user space. Use syscall(2) instead. (Some
architectures, notably ia64, never provided the _syscall macros; on
those architectures, syscall(2) was always required.)
The _syscall() macros do not produce a prototype. You may have to
create one, especially for C++ users.
System calls are not required to return only positive or negative
error codes. You need to read the source to be sure how it will
return errors. Usually, it is the negative of a standard error code,
for example, -EPERM. The _syscall() macros will return the result r
of the system call when r is nonnegative, but will return -1 and set
the variable errno to -r when r is negative. For the error codes,
see errno(3).
When defining a system call, the argument types must be passed by-
value or by-pointer (for aggregates like structs).

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2019-03-06 _SYSCALL(2)